It seems to me the merged product, particularly the 0-2km, highlights nearly every updraft whereas the AzShear seems to do a good job discriminating, and highlights the cell most likely to have strong rotation. In this case the couplet shows up in the western side of the AzShear display and is associated with an embedded rotating updraft, while the AzShear does not highlight the cells further southeast which show weaker rotation. However, the merged product (bottom right) shows bullseyes on all these cells which would make it difficult to triage which storms need attention.

-64BoggsLites

All righty, now let’s take a look at a maturing QLCS mesovortex, starting with AzShear…

At the apex of the bow we see a bit of an AzShear couplet (maximum at the apex with a weaker “blue” minimum just to the south). Comparing this to DivShear…

Hmm, certainly some blue negative divergence (i.e., convergence) there too. Now putting them together into Velocity Gradient…

…things really really start to pop. At this point, it appears that the azimuthal shear is a bit more of a contributor than DivShear. Now looking later when the mesovortex is shifting away from the apex, starting with AzShear, we see a much less focused area…

However, DivShear has really taken up the slack here with a strong convergence signature…

Finally, again putting it all together, we see a very clear signal in the Velocity Gradient showing the best superposition of cyclonic shear and convergence (white area). This is absolutely huge for diagnosing mesovortex evolution since we often see a shifting balance between convergence and azimuthal shear and we can use something like Velocity Potential to get it all at once.

I really appreciate the efforts of Thea and others here who made this dataset available on such short notice. They also showed some examples of these products with tornadic supercells that blew my mind.

#MarfaFront

What’s even better than AzShear?

AzShear + DivShear = Velocity Gradient

Below is a look at AzShear down low with an approaching QLCS. Fairly noisy right?

Here is another view using DivShear – in other words, divergence along a radial similar to how shear is computed across radials. The convergence (i.e., negative divergence) shows up nicely, eh?

Now combining those fields together with Velocity Gradient, things really start to jump.

Look how well DivShear works with MARCs , too:

Certainly better than AzShear when it comes to mid-level convergence…

And then putting them together for Velocity Gradient…



Now, closer to the QLCS “summit”, we see fairly coherent DivShear…

And this is how Velocity Gradient shows it.

Hoping to show some extremely cool results with these fields involving a QLCS mesovortex later in this case if time permits…

#MarfaFront

As I discussed in my previous post , tornadic damage should be focused at the edge of the AzShear maximum. Below is an example from the case we’re studying. The negative AzShear region is not super defined here and I’m wondering if it’s due to excessive noise and/or oversampling from being so close to the radar…

#MarfaFront

Those of us who use GR2Analyst already have access to AzShear via its NROT product. I stated elsewhere that the negative values are important, and here I’ll attempt to explain why.  Below is a schematic of a Rankine vortex, which is akin to a rotating cylinder. Radial velocities are maximized at the edge of the cylinder and then drop off as an inverse  function  of range.

We know azimuthal shear is positive across the entire “cylinder”….

What about just outside the cylinder? If you look closely, you’ll see that there are *negative* AzShear regions on the radials that are just outside of the max/min radial velocities positions…

Putting it all together and thinking of azimuthal shear as a running average of shear as we move across the radials, we get a plot of AzShear that looks something like the black trace below.

We see slightly negative AzShear regions flanking the AzShear maximum, which would be observable for a well-sampled mesocyclone, such as seen on the right hand side of the image below…

Also notice in the figure above that the maximum inbound/outbound velocities line up with where AzShear crosses zero.  Now, let’s pretend we have a rear flank downdraft or RIJ surge by boosting the “inbound” side of the circulation…

Now what does the new AzShear trace look like?

Well, we see more of an AzShear “couplet” as both positive *and* negative shear increase on either side of the surge. The maximum winds are still occurring where AzShear equals zero and this is located directly in between the AzShear maximum and AzShear minimum in conjunction with the center  the surge.

I’m firmly convinced that looking between these two features is an effective way to pinpoint where the most damaging wind will occur, whether it’s tornadic or straight-line. For the smaller tornadoes that most of us get, the southern edge of the AzShear maximum is a sweet spot that has the cumulative effects of rotation, translation, and inflow to maximize winds.

#MarfaFront